For those who (for some reason or another) still have a land line the Netgear Nighthawk AC1900 Cable Modem/ Router (C7100V) is on sale at Amazon [amazon.com]
for only $195.99
has it for the same price.
...as does Frys [frys.com]
If you plan on keeping xfinity and land line Long term it is better than paying the $11 per month for a rental. There may be less expensive options but this one seems to have very good reviews.